Monroe, I'm reading a few more things about this engine, and it may be constant pressure after all... need to do further investigation.
7805's maximum input voltage is around +30V. Any 12V automotive device is supposed to be able to withstand up to 42V big transients. And (big) negative transients also. If you're interested, I can post a link I have here somewhere to a commercial transient Test Equipment where you can see the types of transient normal auto electronics should withstand, and you'll be impressed. There are norms that define these transients.
Concerning the fuse, it is essential also for security. You risk a fire if you don't have one, specially taking in consideration the auto environment and the typical low reliability of home-built prototypes. The place where you're getting the power for your circuit is most probably already protected by a car fuse, by that won't save the device, because those fuses are for lots of Amps and your "thinner" circuit and the wires connecting to it will burn before the "big" fuse.
For the low price of both devices, a transient suppressor and a fuse, I think you unnecessarily risk too much.
I was unable to fully understand what electronics should I have in my circuit to protect against the standard auto transients. Therefore what I did was to add a fuse and a "powerful" voltage transient to each power input (I have 2 inputs). Furthermore, I added a series diode to further protect the voltage regulator against negative spikes and added a big and a small capacitor to the regulator's input. I want to have a rest sleep at night
. My device will be permanently ON. To protect the signal inputs I used 2 common diodes and 2 resistors, plus a capacitor (adds considerable protection against transients) on the injector input. I didn't use zener diodes with resistors just because they are more expensive.
You can add a considerable protection to your microcontroller if you add an extra series resistor after the zener diode. Although a 5.1V zener diode looks perfect, the truth is that they have tolerances like any other devices, that can be up to 10%. This means 5.1 + 10% = 5.6V which is already more than the Vcc+0.5V (5.5V) the micro can handle; not to mention that, as input voltage raises, the regulated voltage will also raise, although slowly. Furthermore, the 7805 also has a tolerance, of +/-4% over manufacturing and temperature (you should take temperature into consideration in an auto circuit, because it will vary a lot), meaning that in some chips and some conditions your Vcc can in fact reach 5V - 4% = 4.8V, worsening the case, now it's 5.6V - (4.8 + 0.5) = 0.3V; with time, ATmega8 internal pin protection diodes will end-up degrading and burning, because current will not be limited on the pin. If you add the series resistor (10K or higher) you'll be protected against this.
Electronics design is fascinating